My Lumia 1020 is not taking pictures of the highest resolution - it's just taking pictures of approximately 5 MP. How do I take a 38mp photo on Lumia 1020?
You need to specify in your camera settings to capture a 5MP + 38MP image, as @Thomas says, by going into your camera app, and setting the "Camera Mode" option to
JPEG (5MP) + DNG (34MP). This takes two images: a 5MP version that is shown to you on the phone, and is what is uploaded to Onedrive if you have that option enabled, and a 38MP version that is hidden.
In order to access the higher-resolution version, you need to connect it to your computer, click on the phone icon, and then go to
Phone > Pictures > Camera Roll. Your high-resolution versions are suffixed with
As for an introduction to the camera features, the user guide is a good starting point.
In the Lumia Camera app (formerly known as Nokia Camera) you can set the resolution it the settings menu. (Screenshot taken with Lumia 930)